Epics and Projects are similar, and there is a relationship between them in that an Epic can be indirectly associated with a Project (or Projects, as the case may be) through the Stories in that Epic.
The difference is that Projects typically map to teams (such as Frontend, Backend, Mobile, Devops, etc), but can represent any open-ended product, component, or initiative. Projects don't have a defined start or end date, and every Story in Clubhouse must be associated with a Project.
We define an individual Epic as a collection of Stories that together make up a larger initiative that your Workspace is working on (and which does have defined start and end dates). Oftentimes an Epic can be a place for multiple Project teams to work together toward a shared goal.